37.5 Endocrine Glands

Endocrine System and Glands

The endocrine system is a network of glands that produce hormones to regulate bodily functions like metabolism, growth, and reproduction. Hormones act as chemical messengers, traveling through the bloodstream to reach target cells. The interplay between different hormones and their feedback mechanisms keeps the body in homeostasis, adjusting to both internal and external changes.

Major Endocrine Glands and Hormones

Each endocrine gland produces specific hormones with distinct roles. Here's a breakdown organized from the brain downward through the body.

Hypothalamus

The hypothalamus is the link between the nervous system and the endocrine system. It controls the pituitary gland by secreting two types of hormones:

• Releasing hormones stimulate the anterior pituitary to secrete its hormones (e.g., thyrotropin-releasing hormone, or TRH, triggers TSH release)
• Inhibiting hormones suppress anterior pituitary secretion (e.g., somatostatin inhibits growth hormone release)

Pituitary Gland

Often called the "master gland," the pituitary has two functionally distinct lobes.

Anterior pituitary produces its own hormones in response to hypothalamic signals:

• Growth hormone (GH) promotes skeletal and soft tissue growth
• Thyroid-stimulating hormone (TSH) stimulates the thyroid gland to produce T3 and T4
• Adrenocorticotropic hormone (ACTH) stimulates the adrenal cortex to release cortisol
• Follicle-stimulating hormone (FSH) promotes follicle development in ovaries and spermatogenesis in testes
• Luteinizing hormone (LH) triggers ovulation and stimulates sex hormone production in both ovaries and testes
• Prolactin (PRL) stimulates milk production in mammary glands

TSH, ACTH, FSH, and LH are called tropic hormones because they act on other endocrine glands rather than on non-endocrine tissues directly.

Posterior pituitary does not make its own hormones. Instead, it stores and releases hormones produced by the hypothalamus:

• Antidiuretic hormone (ADH) increases water reabsorption in the kidneys, concentrating urine and conserving water
• Oxytocin stimulates uterine contractions during labor and milk ejection during breastfeeding

Thyroid Gland

• Triiodothyronine (T3) and thyroxine (T4) regulate metabolic rate, growth, and development. T3 is the more biologically active form.
• Calcitonin lowers blood calcium levels by inhibiting bone resorption (calcium release from bone)

Parathyroid Glands

Four small glands embedded in the back of the thyroid. They secrete parathyroid hormone (PTH), which raises blood calcium levels through three mechanisms:

1. Stimulating osteoclasts to break down bone, releasing calcium
2. Increasing calcium reabsorption in the kidneys
3. Promoting activation of vitamin D, which enhances calcium absorption in the intestines

Adrenal Glands

Each adrenal gland sits on top of a kidney and has two distinct regions.

Adrenal cortex (outer layer) produces three classes of steroid hormones:

• Glucocorticoids (mainly cortisol) regulate metabolism, suppress the immune response, and help the body cope with stress
• Mineralocorticoids (mainly aldosterone) maintain electrolyte balance by promoting sodium reabsorption and potassium excretion in the kidneys, which also affects blood pressure
• Androgens (mainly DHEA) serve as precursors that can be converted into sex hormones like testosterone and estrogen

Adrenal medulla (inner layer) releases epinephrine and norepinephrine during the stress response. These catecholamines increase heart rate, blood pressure, and blood glucose levels, preparing the body for "fight or flight."

Pancreas

The pancreas is both an exocrine and endocrine organ. Its endocrine cells are clustered in the islets of Langerhans, which contain two key cell types:

• Beta cells secrete insulin, which lowers blood glucose by promoting glucose uptake into cells and stimulating glycogen storage in the liver
• Alpha cells secrete glucagon, which raises blood glucose by triggering glycogen breakdown in the liver and releasing glucose into the blood

Gonads

• Ovaries produce estrogens (promote female secondary sexual characteristics and prepare the uterine lining for implantation) and progesterone (maintains pregnancy and prepares mammary glands for milk production)
• Testes produce testosterone (promotes male secondary sexual characteristics and supports spermatogenesis)

Regulation Through Hormone Secretion

Endocrine glands secrete hormones directly into the bloodstream, so hormones can reach cells throughout the body. However, only target cells that express the correct receptor for a given hormone will respond.

When a hormone binds its receptor, it can trigger different types of cellular responses:

• Altered gene expression for steroid hormones that enter the cell and bind intracellular receptors (e.g., cortisol activating anti-inflammatory genes)
• Changes in cellular activity for peptide hormones that bind surface receptors and activate second messenger pathways (e.g., insulin binding causes glucose transporter proteins to move to the cell surface)

The endocrine system regulates a wide range of functions:

• Metabolism: Thyroid hormones set the basal metabolic rate; insulin and glucagon control blood glucose
• Growth and development: GH and thyroid hormones promote growth; sex hormones drive the changes of puberty
• Reproduction: FSH, LH, estrogens, progesterone, and testosterone coordinate reproductive functions
• Homeostasis: ADH maintains water balance; PTH and calcitonin regulate blood calcium; cortisol and insulin influence blood glucose
• Stress response: Cortisol, epinephrine, and norepinephrine mobilize energy and heighten alertness during stress

Roles of Hypothalamus and Pituitary

The hypothalamus is where the nervous and endocrine systems converge. It receives neural input from various brain regions and translates that information into hormonal signals. It communicates with the anterior pituitary through the hypophyseal portal system, a specialized set of blood vessels that carries releasing and inhibiting hormones directly from the hypothalamus to the anterior pituitary.

The anterior pituitary then secretes tropic hormones that act on downstream endocrine glands. The posterior pituitary, by contrast, is really an extension of the hypothalamus: neurons in the hypothalamus synthesize ADH and oxytocin, then transport them down axons into the posterior pituitary for storage and release.

This arrangement creates three major hypothalamic-pituitary axes, each following the same pattern: hypothalamus → anterior pituitary → target gland.

1. HPT axis: Hypothalamus releases TRH → anterior pituitary releases TSH → thyroid gland releases T3 and T4
2. HPA axis: Hypothalamus releases CRH → anterior pituitary releases ACTH → adrenal cortex releases cortisol
3. HPG axis: Hypothalamus releases GnRH → anterior pituitary releases FSH and LH → gonads release sex hormones

Each axis is regulated by negative feedback from the final hormone back to the hypothalamus and pituitary (see below).

Endocrine Signaling and Feedback Mechanisms

Negative feedback is the primary mechanism that keeps hormone levels in a stable range. The end product of a signaling pathway inhibits the earlier steps, preventing overproduction.

Here's how it works with blood glucose as an example:

1. Blood glucose rises after a meal
2. Beta cells in the pancreas detect the increase and release insulin
3. Insulin promotes glucose uptake into cells, lowering blood glucose
4. As blood glucose drops back to normal, the stimulus for insulin release diminishes
5. Insulin secretion decreases

The same logic applies to the hypothalamic-pituitary axes. For instance, in the HPT axis, rising levels of T3 and T4 inhibit both TRH release from the hypothalamus and TSH release from the anterior pituitary. This prevents the thyroid from being overstimulated.

Positive feedback is less common but amplifies a response until a specific event occurs.

The classic example is childbirth: pressure from the baby's head on the cervix triggers oxytocin release, which stimulates stronger uterine contractions, which increases pressure on the cervix, which triggers even more oxytocin. This cycle continues, escalating until delivery, at which point the stimulus is removed and the feedback loop ends.